Ionic Half Equations (Cambridge (CIE) IGCSE Chemistry) : Revision Note

Ionic half equations

Extended tier only

• Electrochemistry is concerned with the transfer of electrons

  • This is why the definitions of oxidation and reduction are in terms of losing or gaining electrons, not oxygen 

  • Oxidation is the loss of electrons 

  • Reduction is the gain of electrons

• As the ions come into contact with the electrode:

  • Electrons are lost or gained

  • The ions form neutral substances

  • These substances are discharged as products at the electrodes

• At the anode, negatively charged ions lose electrons 

  • So, oxidation occurs at the anode

• At the cathode, the positively charged ions gain electrons 

  • So, reduction occurs at the cathode

• Ionic half-equations only show half of what is happening in a reaction involving electron transfer

  • The ionic half-equation for oxidation shows the loss of electrons

  • The ionic half-equation for reduction shows the gain of electrons

• Ionic half-equations must have the atoms and charges balanced 

Writing ionic half-equations

Metals

• Metals are positive ions, e.g. Li⁺, Cu²⁺, Al³⁺ 

• If a metal is produced during electrolysis:

  • The metal ions will gain electrons to form metal atoms

  • The metal ions will be reduced

•  The charge on the metal ion indicates the number of electrons that will be gained

  • For example:

Li⁺ + e^– → Li

Cu²⁺ + 2e^– → Cu

Al³⁺ + 3e^– → Al

Non-metals

• Non-metals are typically negative ions, e.g. Cl^–, Br^–, OH^– 

  • The hydrogen ion, H⁺, is an exception to this

Hydrogen ion / hydrogen half-equation:

• If hydrogen is formed during electrolysis:

  • Two hydrogen ions will gain 2 electrons to form hydrogen, H₂  

  • Hydrogen ions are reduced

2H⁺ + 2e^– → H₂ 

Halide ion / halogen half-equation:

• If a halogen is formed during electrolysis:

  • Two halide ions will lose 2 electrons to form the halogen

  • Halide ions are oxidised

2X^– → X₂ + 2e^– 

Hydroxide ion / oxygen half-equation:

• The ionic half-equation for hydroixde ions forming oxygen is more challenging to balance

  • Hydroxide ions lose electrons to form oxygen, O₂, and water, H₂O

  • Hydroxide ions are oxidised

4OH^– → O₂ + 2H₂O + 4e^– 

Table of reduction and oxidation reactions at the electrodes